Predicting Drug Resistance

The test may provide physicians with a tool to guide patient treatment by predicting if a patient is likely to become resistant to a particular HIV drug, said one of its developers, Feng Gao, M.D., associate professor of medicine at Duke University Medical Center. Drug resistance is one of the most common reasons why therapy for HIV, the virus that causes AIDS, fails. The test, which detects genetic changes, or mutations, in HIV, also may help scientists understand how the constantly evolving virus develops drug resistance, Gao said.

The Duke test examines the genes of HIV strains for mutations at certain positions that are known to be linked to drug resistance. For example, a change at a specific spot along the genetic code - position 46 - of the protease gene results in resistance to the drug indinavir. To assess the test, the researchers analyzed blood samples from three different groups of HIV patients: those who had never received antiretroviral treatment, those who had received treatment but were not currently being treated and those who were receiving treatment but the treatment was not completely successful.

After processing the blood samples and isolating the genetic material in each of them, the researchers added tiny fluorescent tags designed to stick to HIV genes in particular ways. Tags designed to stick to mutated gene locations known to produce drug resistance were labelled to appear green, while tags designed to stick to the same gene locations but where the genes had not mutated were labelled to appear red.

The researchers used a sophisticated computer program to count the number of molecules with green or red fluorescent tags in each sample. The test proved sensitive enough to detect a single mutated virus out of 10,000 nonmutated viruses in the patient samples, Gao said.